86 PHYSICAL SCIENCE 



illustrated in Figs, ii, 12, and 13. In Fig. 11 

 the residual a is seen as white cores within the 

 grey /3, which follows the arrangement of the 

 original a structures, while, in the particular 

 illumination employed, the part that was liquid 

 at the instant of chilling shows as a dark back- 

 ground. In Fig. 12, where the ingot was cooled 

 more slowly, the change has gone farther ; the /5 

 substance ceases to follow the original skeletons 

 of a, a higher magnification brings out the 

 characteristic striated appearance of the 1^, while, 

 owing to a different illumination, the mother 

 liquid shows as a light background. Fig. 13 is 

 taken from an ingot which had been cooled to 

 the same chill point exceedingly slowly, and kept 

 many hours just above that temperature. The 

 whole ingot is now filled with uniform striated 

 /5, a tiny speck of a, seen towards the lower side 

 of the photograph, alone remaining. In the light 

 of these three photographs it is not surprising 

 that the physical and mechanical properties of 

 metals are modified profoundly by differences in 

 the rates at which they have been cooled from 

 a fused condition. 



Following the dotted line in Fig. 10 still 

 further, we see that, in ingots chilled from 

 temperatures about 750°, /5 alone should exist. 

 Fig. 14 shows a chill from 740°, which was 

 cooled to that temperature almost slowly enough 

 to destroy all the primary crystals of a, which 

 now only show as scattered specks of white. 



Again following the dotted line in the equi- 

 librium curve of Fig. 10, we pass the boundary 

 /x, and again enter a region where a and P exist 

 together. The facts on which this curve is 



